Step 1: Understand what is being asked. We have to recall the general outline shape that most of the large continents share when we look at them on a world map.
Step 2: Look at the big continents one by one. South America, Africa and even the Indian peninsula are wide across the top and slowly narrow down to a point at the bottom.
Step 3: Connect this to a simple shape. A shape that is broad at one end and pointed at the other is basically a triangle.
Step 4: Check the other options. A square or a trapezium has straight equal-looking sides, which the continents do not follow. A circular arch is a curved band, again not matching the tapering land masses.
Step 5: Pick the best match. The repeated broad-to-narrow pattern clearly points to a triangular form.
Step 6: State the answer. So the most common shape for continents is a triangle. \[ \boxed{\text{Triangle}} \]